Observations

The Moon hides Jupiter – a challenging daytime occultation

The location of the Sun and Jupiter just after 2pm on 2020 January 23 as seen from Sydney (please ignore the generic Stellarium scenery!).
The location of the Sun and Jupiter just after 2pm on 2020 January 23 as seen from Sydney (please ignore the generic Stellarium scenery!). Image made using Stellarium online.

In its passage through the sky the Moon regularly passes in front of stars and planets, hiding each from view for about an hour. These occultations – ‘to occult’ means ‘to hide’ – can be predicted long in advance. And they provide a powerful sensation of the Moon’s movement about the Earth.

In recent years Australia has seen several occultations of Saturn by the Moon. These often occur at night when they are most easily seen. Sydney Observatory has live streamed a few: 2014 August 04 and 2019 April 18.

Now it’s Jupiter’s turn. On January 23 2020 there will be a daytime occultation of Jupiter by the Moon. This will be more challenging to observe than a nighttime occultation and will require binoculars or a small telescope to view. And it will also require very great care as the Moon will be only 20-degrees away from the Sun – that’s only one hand-span at arms length.

Viewing the Sun through binoculars or a telescope will result in permanent and irreversible eye damage.

When will the occultation happen?

This occultation is visible across New Zealand and the southern half of Australia, however it will be difficult & potentially dangerous to observe from the central and western areas of Australia. The table below provides the times of disappearance and later reappearance of Jupiter for various cities.

A table of times of disappearance and reappearance of Jupiter during its occultation by the Moon on January 23 2020 for cities in Australia and New Zealand.
Table 1: Times of disappearance and reappearance of Jupiter during its occultation by the Moon on January 23 2020 for cities in Australia and New Zealand. Table copied from the “2020 Australasian Sky Guide” by Dr Nick Lomb.

How can I observe this daytime occultation safely?

Daytime occultations are a challenging observation. You will need a pair of binoculars or a small telescope to see this occultation.

Great care must be taken for this observation. Viewing the Sun through binoculars or a telescope, even momentarily, will result in permanent and irreversible eye damage. Your natural blink response does not act fast enough to protect your eyes when using binoculars or telescopes. I do not recommend this observation for children, even under supervision.

Make this observation from within the shadow of a roof, building, awning, carport or other solid unnmoving structure. From the central and western areas of Australia it may be difficult to position yourself in a suitable shadow and I do not recommend this observation if you are in these areas. Do not use the shadow of a tree, umbrella or other movable object.

It is very important that you position your binoculars (or telescope) wholly within the shadow. Ensure the equipment remains in shadow at all times while viewing to prevent any sunlight directly entering the equipment. Monitor the movement of the shadow to ensure your equipment remains within the shadow.

What will I see?

Use the figure above to estimate the location of Jupiter. About five minutes before the disappearance time (Table 1) scan the sky slowly – from within the shadow you have placed yourself in – and locate the crescent Moon. Jupiter will be nearby as in the figure below.

The crescent Moon of 2020 January 23 shortly before occulting Jupiter, as seen from Sydney. Image made using Stellarium online.
The crescent Moon of 2020 January 23 shortly before occulting Jupiter, as seen from Sydney. Image made using Stellarium online.

Once the Moon moves in front of Jupiter there is nothing to see until the reappearance time! However, it is surprising just how quickly Jupiter disappears and how fast the Moon is moving through the sky. Watching occultations by the Moon gives a very direct and graphic sense of the Moon’s movement about the Earth.

Jupiter’s reappearance is more difficult to catch but also more fun as you try to catch it as close to its reappearance time as possible. Again, a few minutes before the tabulated reappearance time locate the Moon and watch for Jupiter to pop into view. The dark edge of the Moon will be invisible during the day so Jupiter will seem to appear out of nowhere!

The crescent Moon of 2020 January 23 shortly after occulting Jupiter, as seen from Sydney. Image made using Stellarium online.
Jupiter reappears from behind the Moon on 2020 January 23. The dark edge of the Moon will be invisible during this daytime occultation so Jupiter will seem to appear from nowhere. As seen from Sydney. Image made using Stellarium online.

So this is a difficult observation. But these occultations are fun to watch and really give a sense of movement in the solar system.

Please take great care to avoid eye damage when observing this daytime occultation of Jupiter. And let us know how you go.

 

 

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